iOS: Properly detect arm64 builds when choosing which -arch flags to pass

Change-Id: Ib410584ba2c1fe342efb18eb955273090d36db8f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>

mkspecs/macx-ios-clang/features/default_post.prf

@@ -195,8 +195,8 @@ macx-xcode {
     QMAKE_MAC_XCODE_SETTINGS += arch_iphoneos arch_iphonesimulator
     unset(QMAKE_XCODE_ARCHS)
 } else {
-# Be more specific about which architecture we're targeting
-    equals(QT_ARCH, arm): \
+    # Be more specific about which architecture we're targeting
+    contains(QT_ARCH, arm.*): \
         actual_archs = $$QMAKE_IOS_DEVICE_ARCHS
     else: \
         actual_archs = $$QMAKE_IOS_SIMULATOR_ARCHS